Short-Term and Long-Term Debt - Short-Term Debt (Details) - Fannie Mae - USD ($) $ in Millions	Dec. 31, 2024	Dec. 31, 2023
Short-Term Debt [Line Items]
Short-term debt of Fannie Mae, outstanding	$ 11,188	$ 17,314
Short-term debt of Fannie Mae, weighted-average interest rate	4.33%	5.13%